How renovations and landscaping quietly defeat termite protection

Many termite problems in established homes don't start with a failed barrier. They start with something an owner or a builder did years after the barrier went in: a garden bed built up against the wall, a new deck, a path poured to the slab edge, or an extension joined to the old house. Each one can open a hidden route past protection that was working perfectly well.

This article explains how termite protection actually works, the common ways renovations and landscaping get around it, and how we plan building work so the protection survives.

How termite protection works

In much of Australia, new houses built where subterranean termites are a known risk need a termite management system. Under the NCC 2022 Housing Provisions, Part 3.4 Termite risk management, the system must comply with AS 3660.1 (or pass the tests in AS 3660.3), and a durable notice has to be fixed to the building in a prominent place, such as the meter box. The notice records the type of system, when it was installed, the life expectancy of any chemical used, and the recommended inspection frequency. Our post on termite management in new homes covers the system types in detail.

The important thing to understand is what these systems are for. The intent in the NCC is to deter termites from getting into a building by a concealed route. As CSIRO's Managing Termites guide puts it, whole-of-building protection uses physical or chemical barriers to deter the concealed access of termites. Even a well-built system won't stop a colony that is determined to reach the timber. What it does is make termites build mud tubes across surfaces where they can be seen, so a regular inspection catches them before the damage gets serious.

That makes the system depend on two things, and both are easy to undo:

• The barrier stays continuous, with no hidden way over, around or through it.
• The inspection zones stay visible, so an inspector can actually see a mud tube if one appears.

Most renovation and landscaping breaches attack one or both.

The common ways it gets breached

Garden beds, mulch and soil against the walls

This is the most common breach we see. A garden bed is built up against the house, mulch is topped up each year, or the ground level rises over time. Soon the soil sits above the slab edge or covers the weep holes in brick veneer. Termites can then travel up behind the soil, unseen, and into the wall cavity.

CSIRO's guidance is direct: avoid gardens directly against external walls, because they raise humidity and conceal termite access, and keep soil and mulch below the concrete slab and away from weep holes. It suggests a concrete strip at least 50 mm lower than the house slab and 300 mm wide to separate the garden from the house and make inspection easier.

Paving, paths and decks that bridge the inspection zone

A new path poured hard up against the slab edge, or a deck or pergola built against the house, can cover the zone an inspector needs to see. The NCC treats attachments such as steps, verandahs, decks, carports, trellises, downpipes, service pipes, hot water units and air-conditioners as needing their own termite management system or an inspection zone between the attachment and the building. An attachment added later, without either, gives termites a covered bridge into the house.

Outdoor timber connected to the soil is a related problem. CSIRO recommends cutting timber posts short of the soil and supporting them on metal stirrups, so pergolas, steps and posts don't give termites a direct path from the ground.

Extensions that join a new slab to an old one

When a new slab is poured against an existing one, the joint between them is a gap that the original barrier was never designed to cover. CSIRO specifically warns that renovations and extensions must not compromise existing barriers, and that the gap where a new slab abuts an older one needs to be sealed or protected with an appropriate termite management system. If the new part of the house has one system and the old part has another, the junction between them has to be designed so there's no break.

New penetrations through the slab

New plumbing, drainage or electrical conduits passing through a slab are a classic concealed entry point. In new construction these penetrations are protected as part of the system. When a renovation cuts new ones, such as a relocated bathroom, a new laundry or a kitchen island with services, they need the same protection, installed by someone who knows which system is already there.

Timber in contact with the ground

Termites find timber in or on the soil quickly. Common culprits around renovations are:

• garden edging and retaining walls made from untreated or low-hazard-class timber, built against the house;
• timber formwork, pegs or off-cuts left in the ground or under a new slab or path (CSIRO also warns against letting timber off-cuts end up in fill);
• timber stored against the house during or after a job.

Each one can feed a colony that then looks for its next meal in the building.

Moisture where it shouldn't be

Termites are drawn to moisture. Leaking taps, air-conditioner condensate lines dripping beside the wall, hot water relief lines, broken downpipes, sprinklers wetting the walls and paving that falls towards the house all create damp spots right where you don't want termites to settle. CSIRO recommends grading ground and paving so rainwater flows away, fixing plumbing leaks, and avoiding sprinklers that regularly wet walls and stumps.

Blocked or crowded subfloors

In houses with suspended timber floors, the subfloor needs to stay ventilated and accessible. The NCC 2022 Housing Provisions, Part 6.2 Subfloor ventilation, require a minimum ground clearance (400 mm where termite inspection is needed, reducing to 150 mm within 2 m of external walls on sloping sites), evenly spaced openings with no dead air spaces, and the subfloor kept clear of debris and vegetation. Renovations often undo this. Vents get covered by a new deck, a garden bed or new paving, and the subfloor fills with stored materials or new ductwork. That leaves a damper subfloor that is harder to inspect, which is exactly the combination termites exploit.

Why this matters more than it seems

None of these breaches looks like a problem on the day. A garden bed looks tidy and a new path looks finished. But they all work the same way: they give termites a hidden route, and they take away the one thing the system relies on, which is being able to see activity early. By the time damage shows inside, in a skirting board that sounds hollow or a door frame that sags, the colony may have been working in the wall for a long time.

That is also why inspections matter, even with a barrier in place. CSIRO notes that barrier integrity can be affected by renovations, garden alterations, slab movement and insecticide depletion, and recommends regular inspections, at least annually. See our post on termite inspections for existing homes for what a good inspection involves.

Questions to ask before renovating or landscaping

1. What termite management system does the house have now? Is the durable notice still in the meter box, and what does it say?
2. Will the work cover the slab edge, weep holes or any inspection zone?
3. Will any new slab, path or deck join or abut the existing building? How will the joint be protected?
4. Will any new pipes or conduits pass through the slab, and who is protecting those penetrations?
5. Will the work block subfloor vents or reduce access underneath?
6. Is any timber going to be in contact with the soil, including edging, posts and formwork?
7. Who is the licensed pest manager advising on this, and will the system be inspected and documented once the work is done?

How we handle it on renovations and extensions

• We check the existing system first. Before we design an extension or a major renovation, we find the durable notice, identify the system, and note where the slab edge, weep holes and inspection zones are.
• We design for continuity. Where new work meets old, whether a new slab, an attached deck or a new path, we plan how the termite management will run across the junction, rather than leaving it to chance on the day.
• We bring in a licensed pest manager where the system needs to be extended, reinstated or treated, so the work is done by someone qualified to do it and the documentation matches.
• We protect penetrations through any new or existing slab, and record them before they are covered. That is part of our hold-point approach.
• We keep inspection zones and subfloor vents clear in our landscaping and paving details, and we don't leave timber formwork, pegs or off-cuts in the ground.
• We hand over the record, including the updated system details, so the next inspector knows what is there.

Termite damage is rarely the result of one big failure. Usually it's a small, well-meaning change that nobody connected to the barrier. Our post on what happens when a renovation uncovers a hidden defect covers what to do if you open up a wall and find damage, and our timber frame article explains why termite management is a design question for every house, not just timber-framed ones.

If you're planning an extension, a new outdoor area or major landscaping and want to keep your termite protection intact, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 3.4 Termite risk management, Australian Building Codes Board
• NCC 2022 Housing Provisions, Part 6.2 Subfloor ventilation, Australian Building Codes Board
• Managing Termites: Understanding termites and reducing their risk to buildings, CSIRO Building Technology Resources

This article is general information, not advice for your property. Termite risk and the right management system depend on your location, construction and existing system, so get advice from a licensed pest manager and confirm requirements with your certifier. Full website disclaimer